Pickling refers to the treatment of metal during processing such as shaping or size reduction, to remove scale and other metal oxides that have accumulated during storage or earlier processing. The treatment consists of exposing the metal to an aqueous bath of strong inorganic acids, typically nitric acid mixed with one or more of sulfuric acid, hydrofluoric acid or hydrochloric acid. As a result of the chemical reactions taking place in the bath, and at times due to thermal decomposition of the acid fumes therein, the atmosphere over the bath contains a high concentration of NOx and possibly SOx. The atmosphere in the pickling tanks is typically regularly purged using a mixture of dry air and nitrogen. This exhaust gas must be treated to significantly reduce NOx, SOx and acid gases before it can be vented to the atmosphere.
NOx refers to the nitrogen oxides commonly present in waste gas effluents. i.e. NO, N.sub.2 O.sub.3, NO.sub.2 and N.sub.2 O.sub.5. These oxides, which have increasing degrees of oxidation in the order given, likewise have varying degrees of solubility/reactivity with aqueous solutions, preferably caustic solutions having a pH between 7 and 14. In general, the higher the oxidation state, the greater the solubility/reactivity with aqueous caustic solutions. It is important to bear in mind that, unlike combustion wherein large percentage of the NOx formed exists as NO, the effluent gas from a pickling operation commonly contains a much higher percentage of higher nitrogen oxides, particularly NO.sub.2. As a general comparison, combustion typically produces at least about 90 vol. percent NO with the balance being NO.sub.2 whereas the effluent gas from a pickling operation typically contains only from about 60 to 65 vol. percent NO with the balance NO.sub.2. These considerations apply to SOx formation as well.
NOx and SOx are currently removed from pickling lines primary by alkaline scrubbing. The use of multiple scrubbings will remove about 40% to 50% by volume of the NOx and SOx in an effluent gas stream as described above. However, more efficient systems were required to meet ever-tightening NOx emission standards. One such system is a wet oxidation system that utilizes hydrogen peroxide or sodium oxychloride in the first scrubber to convert NO to NO.sub.2. The second scrubber contacts NO.sub.2 with a reducing agent, such as sodium hyposulfide, to convert it to nitrogen that can be vented to the atmosphere. This system suffers from high cost and the possibility of release of dangerous emissions, such ClO.sub.2.
Another technology utilized to address the problem of NOx removal is selective catalytic reduction (SCR) which utilizes catalysts to reduce NOx to nitrogen gas. The problem with the use of such systems to treat pickling line effluents is that the HF and H.sub.2 SO.sub.4 fumes must be initially removed, such as in a scrubber. Failure to effectively remove such acid fumes will result in the catalyst becoming plugged or inactivated by the acids. This will result in the discharge, or "slip", of hazardous substances from the system into the environment. SCR and other non-catalytic reduction technologies may also require heating the gas to an optimum temperature for optimal reduction reaction.
Still another technology for the removal of NOx is referred to as low temperature oxidation. This is, in effect, heat removal followed by an ozone-based oxidation system followed by a wet scrubber. The molar ratios utilized for a system are at minimum 1.5 moles of O.sub.3 for each mole of NOx to be removed. Further, in addition to a fairly high rate of ozone consumption, this system requires a series of process steps, such as the heat exchange, which are not required for treatment of the low temperature, saturated emissions from a pickling of metals processing operation.
It would therefore be a significant advance in the art if a process could be provided that would efficiently remove NOx and SOx from gaseous effluents from metal pickling operations. Such a process is provided in accordance with the present invention.